The present invention relates generally to MR imaging and, more particularly, to a method and system of determining and setting excitation pulse parameters for MR data acquisition to minimize repetition time for a given pulse sequence without exceeding prescribed electromagnetic energy exposure levels and RF heating limits.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals is digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
In prescribing an MR scan, a number of parameters must be determined to effectively acquire MR data such that a diagnostically valuable MR image can be reconstructed. These parameters include the type of imaging protocol to be used, spin echo or gradient echo, flip angles, single slice or multi-slice acquisition, and k-space filling scheme. Another parameter that also must be considered is the repetition time (TR) of the pulse sequence that will be applied for MR data acquisition. The TR of an MR scan defines how frequently a pulse sequence will be applied to image a particular region-of-interest (ROI).
Generally, it is desirable to conduct an MR scan with the shortest possible TR so as to expedite the MR data acquisition process. In this regard, acquiring MR data expeditiously reduces the likelihood of subject discomfort in the MR scanner, which reduces the likelihood of subject-induced motion artifacts, mis-registration of MR data, and subject fatigue during breath-hold applications, as well as increases patient throughput. Accordingly, the conventional approach for optimizing short TR pulse sequences is to use the maximum available excitation field strength to achieve the shortest RF duration for a given pulse shape and flip angle. The maximum available excitation field strength is typically a function of the RF coil and amplifier systems of the MR scanner. As such, a pulse sequence having a given pulse shape and flip angle will not be repeated at a repetition time shorter than that supported by the MR scanner hardware.
Additionally, it has been shown that for high-field applications, relying upon the maximum available excitation field strength for pulse sequences requiring short TR and large flip angles is less than optimal. This is a result of the minimum TR being constrained by specific absorption rate (SAR) limitations. As such, for high-field pulse sequences having short TR and large flip angles such as FIESTA and ce-MRA (SPGR), the minimum achievable repetition time (TRmin) is predominantly limited by SAR constraints resulting in a repetition time longer in duration than the minimum repetition time supported by the MR scanner or desired for the given pulse sequence.
The SAR of an MR scan quantifies the amount of electromagnetic energy to which a subject is exposed during the MR scan. SAR can be generally defined as a function of flip angle of a given pulse sequence which depends on the excitation field strength and the duration of the prescribed excitation pulse as set forth in the following expression:SAR≅W≅ωB12τ=γB02Θ2τ/(ηγτ)2=B02Θ2/η2τ,where W=energy deposit per pulse, Θ=flip angle, B0=the static field strength, B1=RF magnetic field strength, τ=RF pulse duration, η=waveform factor, γ=gyro-magnetic ratio.
From the above expression, it is clear that the amount of RF energy deposited during an MR scan increases by a factor of two in proportion to the excitation field strength. For instance, the SAR for an MR scan conducted at 3 T is four times the SAR of an MR scan conducted at 1.5 T. As such, pulse sequences requiring large flip angles and short TRs, while possible at 1.5 T, may be unachievable at higher magnetic field strengths, such as 3 T, because the desired TR at the high field is not feasible. That is, the minimum TR required for the high-field pulse sequence may require an excitation field strength that results in RF exposure and/or RF heating that exceeds a maximum industry-permitted SAR for the given MR scan. One skilled in the art will appreciate that the maximum SAR levels are generally set by government regulatory agencies such as the Food and Drug Administration in the United States.
One approach to reduce the minimum achievable repetition time for a given pulse sequence is to reduce the flip angle of the excitation RF pulse so as to reduce the SAR level for that pulse sequence. However, simply reducing the flip angle of the RF pulse might not be desirable given that alteration in signal-to-noise and/or contrast-to-noise might result in less than optimal image quality. Additionally, simply lowering the maximum available B1 strength or magnitude of the RF pulse can degrade pulse sequence performance and, hence, negatively affect image quality severely if not performed appropriately.
It would therefore be desirable to design a method and system of judiciously and automatically setting the maximum available excitation field strength for MR data acquisition to minimize TR for the pulse sequence without exceeding prescribed SAR and RF heating limits.